Extruders capable of extremely efficient degassing are required for use in the preparation of bulk plastics materials, both for legal reasons and for the reasons of economy. Hence, the input melt material input must be degassed from a gas content of up to 15% to a content of less than 500 ppm when polymerizing the melt.
In U.S. Pat. No. 3,992,500 there is disclosed a typical known multi-stage degassing extruder in which a free volume is created under each degassing port by reducing the diameter of the screw core. So-called "metering zones", in which the screw core has a larger diameter in the region of the degassing ports, are provided downstream of the degassing zone so that, in these regions the melt is subjected to a high pressure, which pressure is lowered in the following degassing sections to permit and/or to cause degasification of the melt. The screw, in such an arrangement, must also have a large thread depth in the melt intake regions. In order to be able to build up a material pressure, the diameter of the screw core is enlarged in the metering zones which are connected downstream of the degassing zones but upstream of an adjacent intake section.
The manufacture of a screw of this type is very costly because it is necessary to provide the screw with core portions of greatly differing diameters each of which portions extends over an accurately predetermined longitudinal length of the screw. These lengths are of importance because a different conveying capacity is required of the screw in each degassing stage.
The rotational speed of the screw must be kept within a specific range. This is in order to prevent too large a quantity of melt from being conveyed into the degassing regions and hence being caused to enter into and through the degassing ports.
If, for example, it is calculated that the screw threads should only be half filled in the first degassing zone in order to obtain satisfactory degassing, a predetermined low rotational speed of the screw within the range must be maintained. At the same time, however, the depths of the thread in the subsequent sections of the screw must be designed to cope with a predetermined conveying capacity, which conveying capacity is dependent upon the prevailing number of screw threads, the depth of the screw thread, the pitch of the screw flight and the rotational speed. Such design is necessary to ensure that the essentially partial filling of the screw threads in the subsequent degassing section having deeply cut screw threads is achieved.
It is possible to commence with partial filling of the screw below the respective degassing ports if a predetermined ratio of the conveying capacities in the individual metering and degassing sections has been set. This ratio is determined by the appropriate design of the screw geometry. However, this calculated ratio of the conveying capacities is of little or no use when the rotational speed of the screw lies outside the above-referenced range.
Moreover, this calculated ratio for optimum operation of a degassing extruder also becomes of little use when one attempts to degas a material having a different melt-flow index, that is to say, having a different viscosity, in the extruder.
The rotational speed of the screw must be changed in order that the extruder is not restricted to degassing melts having specific, relatively narrow ranges of viscosities, assuming that there is no desire to alter construction of the screw once this has been established. The partial filling of the screw threads below the degassing ports, which is an optimum requirement for satisfactory degassing, is maintained, but only in one stage. The other degassing stages must then be operated with a lower than optimum filling of the screw threads. If this requirement is ignored, the melt passes into and through the degassing ports.
Although such a measure enables materials having different viscosities to be degassed by the same extruder, the user must accept that the output of the extruder is considerably reduced, since it is in only one stage that operation is carried out with the screw threads filled with the melt to an optimum degree. However, a reduction in the output of the degassing extruder is unsatisfactory. For the purpose of maintaining satisfactory degassing and to prevent the melt from passing into and through the other degassing ports, it is necessary to operate at a different screw rotational speed and hence also with a greatly differing output.